Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

Lai Jiangshan
2009-04-21T06:55:29Z

Stephen Hemminger wrote:
>> xt_info_rdlock_bh() called recursively here will enter the
>> critical region without &__get_cpu_var(xt_info_locks)->lock.
>
> NO spin_lock_bh always does a preempt_disable
>
> xt_info_rdlock_bh (depth = -1)
> +1 preempt_disable
> spin_lock_bh
> +1 preempt_disable
> -1 preempt_enable_no_resched
> ---
> +1
>
> Second call preempt_count=1 (depth = 0)
> xt_info_rdlock_bh
> +1 preempt_disable
> -1 preempt_enable_no_resched
> ---
>

I think I had made you mistook my email.

The preempt_count is correct, but I thought about lock-save:
we must hold &__get_cpu_var(xt_info_locks)->lock
when we enter the read-side critical region.

--------------------
xt_info_rdlock_bh() (depth = -1)
preempt_disable()
depth++
==========>interrupt here
==========>
==========>xt_info_rdlock_bh() (depth = 0)
==========> preempt_disable()
==========> depth++
==========> preempt_enable_no_resched()
==========>
==========>enter the read-side critical region *without* lock.
==========> it may get trashy data.
==========>
==========>xt_info_rdunlock_bh()
==========>
==========>interrupt return.
spin_lock_bh()
preempt_enable_no_resched()

enter the read-side critical region *with* lock.

xt_info_rdunlock_bh().
----------------------

So I asked:
> +void xt_info_rdlock_bh(void)
> > +{
> > + struct xt_info_lock *lock;
> > +
> > + preempt_disable();
> > + lock = &__get_cpu_var(xt_info_locks);
> > + if (likely(++lock->depth == 0))

So what happen when xt_info_rdlock_bh() called recursively here?

> > + spin_lock_bh(&lock->lock);
> > + preempt_enable_no_resched();
> > +}
> > +EXPORT_SYMBOL_GPL(xt_info_rdlock_bh);
> > +

----------
Is this OK? (Now I suppose we can enter the read-side critical region
in irq context)

void xt_info_rdlock_bh(void)
{
unsigned long flags;
struct xt_info_lock *lock;

local_irq_save(flags);
lock = &__get_cpu_var(xt_info_locks);
if (likely(++lock->depth == 0))
spin_lock_bh(&lock->lock);
local_irq_restore(flags);
}

Lai